The present invention relates generally to the field of electrical power conversion, and more particularly to soft switched power converters equipped with synchronous rectification. Even more particularly, the present invention relates to zero voltage switching (ZVS) power conversion implementing a commutation current boosting method.
Soft switched power converters, such as those configured for switching at zero voltage (ZVS), are popular for their relatively low switching losses and smooth switching waveforms, providing high efficiency and good electromagnetic compatibility (EMC). One of the parameters to be calculated during design of such converters is a level of commutation energy required to maintain zero volt switching. This minimum level of commutation energy can be calculated based on parasitic elements in the circuit design. From this calculation, a value for the minimum amplitude of the commutation current can be obtained. This current introduces an internal reactive power constantly circulating throughout the converter, and is responsible for a remarkable degree of power losses. These losses further result in lower efficiency, especially at the lower half of the load range.
Therefore, designers try to minimize the level of the commutation current, so as to offer a smaller reactive energy circulating in power circuits of the converter and also resulting in a smaller permanent power loss and higher efficiency. Unfortunately, a smaller commutation current also notably reduces ZVS reliability.
One previously known method for addressing this problem involves implementing a variable dead time in combination with a variable phase shift between primary and secondary control signals generated by a controller, thereby ensuring that power switching elements are not exposed to cross conduction or non-ZVS operation across the range of operating conditions. This approach, unfortunately, brings about additional demands on the controller, ultimately resulting in any or all of a more expensive, more complex or lower performing control solution.
Conventional power converters further typically enable or disable synchronous rectifier switches as a function of the load current. One problem with this technique is that it may cause dips on the output voltage, which can be critical for applications requiring tight output voltage regulation. The need for current sensing in order to provide proper synchronous rectifier control further requires a more demanding design.